MOS transistor having improved oxynitride dielectric

ABSTRACT

High quality ultrathin gate oxides having nitrogen atoms therein with a profile having a peak at the silicon oxide-silicon interface are formed by oxidizing a surface of a monocrystalline silicon body in an atmosphere of nitrous oxide (N 2  O) at a temperature above 900° C. preferably in the range of 900°-1100° C., and then heating the silicon body and oxidized surface in an atmosphere of anhydrous ammonia to introduce additional nitrogen atoms into the oxide and increase resistance to boron penetration without degrading the oxide by charge trapping. The resulting oxynitride has less degradation under channel hot electron stress and approximately one order of magnitude longer lifetime than that of conventional silicon oxide in MIS applications.

This is a Division of application Ser. No. 08/179,016, filed Jan. 7, 1994, now U.S. Pat. No. 5,397,720.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor processing technology and more particularly the invention relates to dielectrics used in submicron devices as used in ULSI microelectronic circuits.

The metal-insulator-silicon (MIS) transistor including the metal-oxide-silicon (MOS) transistor is used in large scale integrated (LSI), very large scale integrated (VLSI), and ultra large scale integrated (ULSI) microelectronic circuits. The transistor has a current carrier source region formed in a surface of a semiconductor (e.g., silicon) body, a carrier drain region formed in the surface and spaced from the source, and between the source and drain is a channel region through which the current carriers flow. Overlying the channel region and aligned with edges of the source and drain is a gate electrode which is physically and electrically separated from the channel by a dielectric layer. Typically the dielectric comprises a silicon oxide (SiO₂).

To provide stress relief and to enhance the electrical qualities of the dielectric, a silicon nitride (Si₃ N₄) layer has been placed over the silicon oxide layer between the gate electrode and the channel region. Additionally, nitride ions have been placed in the silicon oxide layer by ion implantation and by NH₃ (anhydrous ammonia) nitridation.

However, NH₃ nitridation incorporates hydrogen in the oxides, which increases electron trapping significantly. Reoxidation of nitrided oxides (ROXNOX) is unable to eliminate the nitridation induced electron traps, leading to worse hot carrier reliability of p-MOSFETs. Even in n-MOSFETs with ROXNOX SiO₂, the hot carrier reliability is degraded under peak I_(g) stress due to the presence of electron traps. Although the electron trapping problem can be reduced by using a very light nitridation, the resulting oxides do not show sufficient resistance to boron penetration. Consequently, (reoxidized) NH₃ -nitrided gate oxides may not be adequate for p⁺ -polysilicon gated p-MOSFETs. On the other hand, alternatives to (reoxidized) NH₃ -nitrided oxides, namely N₂ O-nitrided and N₂ O-grown oxides may not contain sufficient nitrogen atoms to prevent boron penetration.

The present invention is directed to an improved dielectric for use in deep submicron MOS devices and the fabrication thereof.

SUMMARY OF THE INVENTION

In accordance with the invention, a silicon oxide dielectric is grown in a nitrous oxide (N₂ O) environment and then nitrided in an anhydrous ammonia (NH₃) atmosphere to introduce a sufficient amount of nitrogen at the silicon/silicon oxide interface to provide resistance to boron penetration but with reduced electron traps induced from the nitridation as compared to pure silicon oxide which has been nitrided.

In fabricating the dielectric, a silicon surface is oxidized in an atmosphere consisting essentially of nitrous oxide (N₂ O). In preferred embodiments, a pure (e.g., 99.998%) nitrous oxide gas is employed at a temperature of approximately 900°-1100° C. Ultrathin (approximately 60-65 Å) gate dielectrics can be grown having a nitrogen rich layer at the silicon/silicon oxide interface. Thereafter, the structure is placed in an atmosphere of NH₃ at a temperature in the range of 800° C.-950° C. for a period of time between 30 seconds and 30 minutes; to introduce additional nitrogen atoms. The structure is then annealed in a nitrogen gas (N₂) atmosphere.

Results show that NH₃ -nitrided N₂ O-oxides have excellent electrical (low N_(f)) and reliability properties (smaller charge trapping and suppressed interface state generation) comparable to N₂ O oxides, with an additional advantage of significantly improved resistance to boron penetration.

The invention and objects and features thereof will be more readily apparent from the following description and appended claims when taken with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of an MIS transistor including a dielectric in accordance with the invention.

FIG. 2 is a flow diagram of a process of forming a dielectric in the device of FIG. 1 in accordance with the invention.

FIG. 3 illustrates changes in gate voltage required to maintain a constant current density of -100 mA/cm² in MOS capacitors (area: 5×10⁴ cm²) with different gate dielectrics.

FIG. 4 illustrates Weibull plots of time-to-breakdown in MOS capacitors (area: 5×10⁻⁵ cm²) with different gate dielectrics, measured at a current density of 100 mA/cm² under positive gate bias.

FIG. 5 illustrates increase in midgap interface state density (ΔD_(it-m)) in MOS capacitors (area: 5×10⁻⁴ cm²) with different gate dielectrics, caused by F-N injection at a current density of 2 mA/cm² with positive gate bias.

FIG. 6 illustrates flatband voltage in BF₂ -implanted p⁺ -polysilicon gate MOS capacitors with different gate dielectrics after N₂ anneal at 900° C. for 30 min.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to the drawing, FIG. 1 is a section view of a MISFET transistor including a dielectric in accordance with the invention. The transistor is formed in a silicon substrate 10 with a carrier source region 12 and a carrier drain region 14 formed in a major surface of the substrate. The source and drain are separated by a channel region 16 through which current carriers flow. Control of the conduction of the carriers through the channel 16 is achieved by voltage biasing a gate electrode 18 spaced above the channel region in alignment with edges of the source and drain and separated from the substrate by a dielectric 20. In an enhancement mode transistor, the channel 16 is nonconducting until a voltage bias is applied to induce a conductive channel under the dielectric.

In ultra large scale integrated (ULSI) microelectric circuits the transistor of FIG. 1 is fabricated according to submicron design rules in which the channel region 16 will be less than one micron in length. Because of the small dimensions, high quality ultrathin gate oxides are required.

In accordance with the present invention, an oxynitride dielectric layer having enhanced dielectric characteristics and a simple method for forming same are provided. The oxynitride layer includes a nitrogen profile extending from the interface of the oxynitride layer and underlying silicon substrate. The oxidation rate in forming the oxynitride layer is much slower than that of a conventional silicon oxide formed in a pure oxygen ambient, thus control of the layer thickness is easier to maintain. The nitrogen concentration increases with increasing temperature. The oxynitride layer is then nitrided in an atmosphere of NH₃ and annealed in an atmosphere of N₂.

FIG. 2 illustrates a flow diagram in the process for forming the oxynitride dielectric. As is described in Hwang et al., "Electrical Characteristics of Ultrathin Oxynitride Gate Dielectric Prepared by Rapid Thermal Oxidation of Si in N₂ O," Applied Physics Letters 57(10), 3 Sep. 1990, the surface of the silicon substrate on which the oxynitride is to be grown is initially cleaned using a conventional RCA wet chemical process. Thereafter, the substrate is placed in an atmosphere of essentially pure nitrous oxide. With the temperature raised to 900°-1100° C. oxidation of the silicon substrate occurs with a nitrogen gradient extending upwardly in the dielectric layer from the silicon/silicon oxide interface. Thickness of the dielectric layer is readily controlled since the oxidation of the silicon surface is at a much lower rate than oxidation in a pure oxygen ambient.

In accordance with the invention, the oxide dielectric layer is then heated in an atmosphere of anhydrous ammonia (NH₃) to increase the nitrogen atom content of the dielectric to prevent boron penetration and with reduced charge trapping and suppressed interface generation. The ammonia is introduced at atmospheric pressure and then heated to a temperature in the range of 800° C.-1100° C. preferably between 850° C.-950° C. for a time period between 30 seconds and 30 minutes.

After nitriding of the dielectric layer, thermal annealing is performed in a nitrogen (N₂) ambient for a short period of time (e.g., 60 seconds) at the oxidation temperature to reduce fixed charge density. The above steps can be carried out in-situ in a rapid thermal system without removing the wafer from the chamber. Thereafter the gate electrode is formed by depositing polysilicon which is then doped. Finally, the source and drain regions are formed by implant followed by annealing in dry oxygen. Aluminum contacts can then be deposited by sputtering and patterned to complete the interconnection in the integrated circuit.

The invention has been verified using MOS capacitors fabricated on 3-5 Ω-cm (100) Si substrates. Control and N₂ O-oxides (-80 Å, measured by ellipsometry and C-V) were grown at 950° C. in a resistance heated furnace in O₂ and N₂ O ambients, respectively. Some samples with control and N₂ O-oxides were nitrided in NH₃ for different durations at 900° C., followed by N₂ annealing at 950° C. for 20 min. SIMS measurements indicated a significant increase in interfacial [N] by NH₃ -nitridation (e.g., [N]˜10% after 5 min. NH₃ -nitridation. Electrical and reliability properties were studied on POCl₃ -doped n⁺ -polysilicon gated MOS capacitors (gate areas: 5×10⁻⁴, 5×10⁻⁵ cm²) fabricated on p-type substrate. P⁺ -polysilicon gates for capacitors on n-type substrate were prepared by BF₂ -implant (5.5×10¹⁵ cm⁻², 50 KeV) followed by drive-in anneal in N₂ at 900° C. for 30 min. These capacitors (areas: 5×10⁻⁴ cm²) wee used to study boron penetration.

Fixed charge (N_(f)) and midgap interface state density (D_(it-m)) were extracted from C-V data on capacitors with various gate dielectrics. Unlike the large increase (˜2×10¹¹ cm⁻²) in N_(f) by 5 min NH₃ -nitridation of pure oxides, the increase in N_(f) by NH₂ -nitridation of N₂ O-oxides is significantly smaller (˜7×10¹⁰ cm⁻²) . D_(it-m)) values in the range of 4-7×10¹⁰ eV⁻¹ cm⁻² were observed for NH₃ -nitrided N₂ O-oxides.

Charge trapping properties of NH₃ -nitrided N₂ O oxides were studied by monitoring the changes in gate voltage (ΔV_(g)) to maintain a constant current density (-100 mA/cm²) in MOS capacitors. As shown in FIG. 3, NH₃ -nitrided pure oxides show a positive ΔV_(g), indicating significant electron trapping by nitridation induced traps. Nevertheless, even after 20 min NH₃ -nitridation, N₂ O-oxides show only a small increase in electron trapping. ΔV_(g) in NH₃ -nitrided pure oxides does not show a saturating trend, indicating a significant amount of high-field induced trap generation. On the contrary, despite a severe NH₃ -nitridation, the trap generation rate in N₂ O-oxides is negligible. Consequently, even after a severe NH₃ -nitridation, N₂ O-oxides still show superior charge trapping properties compared to conventional oxides. Thus, it is possible to fabricate p-MOSFETs using these gate dielectrics, which show superior hot-carrier reliability, unlike p-MOSFETs with reoxidized NH₃ -nitrided oxides which show worse reliability than those with conventional gate oxides. These observations are quite significant, because they imply that the trade-off between sufficient [N] for better barrier properties vs. worse electron trapping can be greatly alleviated by using NH₃ -nitridation of N₂ O-oxides.

FIG. 4 shows Weibull plots for time-to-breakdown (t_(bd)) in n⁺ -polysilicon gated capacitors with different gate dielectrics. A large degradation in t_(bd) is observed in NH₃ -nitrided SiO₂ due to 5 min NH₃ -nitridation. The degradation in t_(bd) is usually attributed to large electron trapping in these oxides, which is apparent from FIG. 3. Contrary to such significant degradation in t_(bd) by NH₃ -nitrided of pure oxides, 20 min NH₃ -nitridation actually increased t_(bd) in N₂ O-oxides to some extent. NH₃ -nitridation for 5 min degraded charge-to-breakdown Q_(bd) (50% failure @ 100 mA/cm²) in pure oxides from 50 C/cm² to 10.4 C/cm², while 20 min NH₃ -nitridation improved Q_(bd) from 90 C/cm² to 118 C/cm² in N₂ O-oxides.

Interface state generation (ΔD_(it)) in these devices due to constant current stress if plotted in FIG. 5. NH₃ -nitridation increases ΔD_(it) in pure oxides. However, ΔD_(it) in NH₃ -nitrided N₂ O-oxides is comparable to the N₂ O-oxides. The large Δd_(it) in NH₃ -nitrided pure oxides is attributed to a distorted interfacial region due to the formation of mismatched Si--N bonds in the Si--O network. However, as seen from FIG. 5, nitrogen incorporation in N₂ O-oxides by NH₃ -nitridation does not lead to such undesirable effect. This observation suggests nitrogen incorporation at the interface by NH₃ -nitridation of N₂ O-oxides is somewhat different from that in pure oxides.

To study the barrier properties against boron penetration, flatband voltages (V_(fb)) were monitored for p⁺ -polysilicon gate capacitors with various dielectrics. Since the penetrated active boron acts as a thin sheet of negative oxide charge at the Si/SiO₂ interface, the extent of penetration can be estimated from V_(fb) measurements. As shown in FIG. 6, after a 30 min drive-in anneal in N₂ at 900° C. positive V_(fb) values are observed for all capacitors, indicating boron penetration in all these devices. The oxide negative charge due to boron penetration, estimated from the V_(fb) values and corrected for N_(f), was 9.8×10¹², 2.3×10¹², 5.6×10¹² and 3.0×10¹² cm⁻² for pure oxide, NH₃ -nitrided pure oxide, N₂ O-oxide and NH₃ -nitrided N₂ O-oxide, respectively. As can be seen, boron penetration in NH₃ -nitrided N₂ O-oxides is comparable to NH₃ -nitrided pure oxides, but is less than that in N₂ O-oxides, which, by itself, shows better resistance to boron penetration than pure oxides.

There has been described a new process of NH₃ -nitridation of N₂ O-oxides to introduce sufficient amount of nitrogen at the Si/SiO₂ interface. This resulting dielectric has excellent electrical and reliability characteristics, without exhibiting the undesirable side-effects as in the case of NH₃ -nitridation of pure oxides. Moreover, an improved resistance to boron penetration is achieved due to sufficient nitrogen at the interface, which is desirable for the fabrication of p⁺ -polysilicon gated surface channel p-MOSFETs.

While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A MIS transistor comprisinga body of monocrystalline silicon having a major surface, a source region and a drain region formed in said body and abutting said major surface, said source region and said drain region being spaced apart with a channel region therebetween, an oxynitride layer on said major surface over said channel region, said oxynitride layer being heated in an atmosphere of anhydrous ammonia to introduce nitrogen atoms with said nitrogen atoms having a concentration profile with a peak at the oxynitride-silicon interface, and a gate electrode on said oxynitride layer over said channel region, said oxynitride layer extending continuously from said gate electrode to said channel region.
 2. The MIS transistor as defined by claim 1 wherein said oxynitride layer is formed by oxidizing said major surface in an atmosphere of nitrous oxide.
 3. The MIS transistor as defined by claim 2 wherein said major surface is oxidized at a temperature above 1000° C.
 4. The MIS transistor as defined by claim 2 wherein said major surface is oxidized at a temperature of 900°-1100°C.
 5. This MIS transistor as defined by claim 1 wherein said nitrogen atoms are introduced in said atmosphere of anhydrous ammonia at a temperature of 800° C.-1100° C. 